When drilling or coring holes in sub-surface formations, it is sometimes desirable to be able to vary and control the direction of drilling, for example to direct the borehole towards a desired target, or control the direction horizontally within the payzone once the target has been reached. It may also be desirable to correct for deviations from the desired direction when drilling a straight hole, or to control the direction of the hole to avoid obstacles.
"Rotary drilling" is defined as a system in which a downhole assembly, including the drill bit, is connected to a drill string which is rotatably driven from the drilling platform. The established methods of directional control during rotary drilling involve variations in bit weight, r.p.m. and stabilisation. However, the directional control which can be exercised by these methods is limited and conflicts with optimising bit performance. Hitherto, therefore, fully controllable directional drilling has normally required the drill bit to be rotated by a downhole motor, either a turbine or PDM (positive displacement motor). The drill bit may then, for example, be coupled to the motor by a double tilt unit whereby the central axis of the drill bit is inclined to the axis of the motor. During normal drilling the effect of this inclination is nullified by continual rotation of the drill string, and hence the motor casing, as the bit is rotated by the motor. When variation of the direction of drilling is required, the rotation of the drill string is stopped with the bit tilted in the required direction. Continued rotation of the drill bit by the motor then causes the bit to drill in that direction.
The instantaneous rotational orientation the motor casing is sensed by survey instruments carried adjacent the motor and the required rotational orientation of the motor casing for drilling in the appropriate direction is set by rotational positioning of the drill string, from the drilling platform, in response to the information received in signals from the downhole survey instruments. A similar effect to the use of a double tilt unit may be achieved by the use of a "bent" motor, a "bent" sub-assembly above or below the motor, or an offset stabiliser on the outside of the motor casing. In each case the effect is nullified during normal drilling by continual rotation of the drill string, such rotation being stopped when deviation of the drilling direction is required.
Although such arrangements allow accurately controlled directional drilling to be achieved, using a downhole motor to drive the drill bit, there are reasons why rotary drilling is to be preferred.
Thus, rotary drilling is generally less costly than drilling with a downhole motor. Not only are the motor units themselves costly, and require periodic replacement or refurbishment, but the higher torque at lower rotational speeds permitted by rotary drilling provide improved bit performance and hence lower drilling cost per foot.
Also, in steered motor drilling considerable difficulty may be experienced in accurately positioning the motor in the required rotational orientation, due to stick/slip rotation of the drill string in the borehole as attempts are made to orientate the motor by rotation of the drill string from the surface. Also, rotational orientation of the motor is affected by the wind-up in the drill string, which will vary according to the reactive torque from the motor and the angular compliance of the drill string.
Accordingly, some attention has been given to arrangements for achieving a fully steerable rotary drilling system.
For example, Patent Specification No. WE090/05235 describes a steerable rotary drilling system in which the drill bit is coupled to the lower end of the drill string through a universal joint which allows the bit to pivot relative to the string axis. The bit is contra-nutated in an orbit of fixed radius and at a rate equal to the drill string rotation but in the opposite direction. This speed-controlled and phase-controlled bit nutation keeps the bit heading off-axis in a fixed direction. Such arrangement requires the provision of a controlled servo of high power.
British Patent Specification No. 2246151 describes an alternative form of steerable rotary drilling system in which an asymmetrical drill bit is coupled to a mud hammer. The direction of the borehole is selected by selecting a particular phase relation between rotation of the drill bit and the periodic operation of the mud hammer.
British Patent Specifications Nos. 2172324 A, 2172325 A and 2177738 A (Cambridge Radiation Technology Limited) disclose arrangements in which lateral forces are applied to a drilling tube above the drill bit so as to impart a curvature to the drilling tube and thereby control the drilling direction. Such arrangements are complex and require large downhole assemblies.
U.S. Pat. No. 4,995,465 (J. L. Beck and L. D. Taylor) describes a rotary drilling system in which a bent-sub is connected behind the drill bit so that the bit extends angularly with respect to the drill rod. An actuator, such as an hydraulic ram, is provided at the surface for exerting thrust on the end of the drill rod which is transmitted along the rod to the drill bit. The thrust applied axially along the drill rod is pulsed to effect the desired trajectory of the drilling, the pulsing of the drill rod being based upon signals received from a downhole monitor.
U.S. Pat. No. 4,637,479 (L. J. Leising) describes a roller-cone bit carried on a drilling tool in which a rotating flow-obstructing member controls the flow of drilling fluid to discharge passages in the drill bit. By controlling the rate of rotation of the flow obstructing member, drilling fluid may be sequentially discharged from the bit passages into only a single peripheral sector of the borehole, thereby diverting the drill bit into a different path by eroding the formation in that sector.
our British Patent Application No. 9023465.7 refers to the use of an hydrostatic bearing, for example in the gauge section of a drill bit, to provide low-friction engagement between a bearing pad and the wall of the borehole. Such a low-friction bearing pad is required in certain arrangements for reducing or eliminating bit whirl.
U.S. Pat. No. 4,416,339 discloses a device for effecting deviation of a drill bit during rotary drilling, the device comprising a hinged paddle which may be urged outwardly from the drill string and toward the wall of the borehole by operation of a piston and cylinder device. Flow of fluid to and from the piston and cylinder device is controlled by an oscillating gate means which is responsive to the attitude and rotation of the bottomhole assembly, and is not positively controlled in synchronism with rotation of the drill bit.
U.S. Pat. No. Re. 29,526 discloses an arrangement where part of the bottomhole assembly comprises an external sleeve above the drill bit which is displaceable laterally by selectively inflating and deflating fluid filled bladders arranged around the inner periphery of the sleeve, the inflation and deflation of the bladders, and hence displacement the sleeve, being controlled in accordance with the orientation of a non-rotating pendulum mounted in the drill pipe.